The present invention concerns a process for making aqueous colloidal polytetrafluoroethylene (PTFE) dispersions, obtained by emulsion polymerization of tetrafluoroethylene in the presence of the lithium salt of a perfluoroalkanesulfonic acid as the emulsifying agent, to give the products made from the PTFE the desired qualities
According to known processes, aqueous colloidal dispersions of PTFE are prepared in a pressure-resistant autoclave by placing the gaseous monomer, or a mixture of monomers in the case of the preparation of copolymers, in contact with an aqueous solution containing an initiator capable of forming free radicals at the polymerization temperature, possibly a buffer for keeping the medium at a given pH, and an emulsifier which generally is an alkaline salt or ammonium salt of a carboxylic or sulfonic acid whose carbon chain possesses only fluorine atoms and possibly chlorine atoms. The polymerization is initiated by free radicals originating from the decomposition of a water-soluble peroxide such as succinyl peroxide or of an alkaline persulfate or ammonium persulfate. The persulfate is used alone if the polymerization temperature is above approximately 50.degree. C., or in association with a reducing agent such as ferrous salt, silver nitrate, sodium bisulfite in the contrary case. The pH can be kept alkaline by a buffer, such as an alkaline or ammonium phosphate, pyrophosphate, or borate.
The polymerization temperature generally is between 5.degree. C. and 100.degree. C., and the pressure between 1 and 100 bars, preferably between 10 and 25 bars.
The qualities of the finished products manufactured from PTFE powder such as, for instance, tubing, wire coating, rods, strips (tapes), depend on the method used for the production of the aqueous colloidal PTFE dispersion. In particular, the nature of the ingredients used in polymerization can have an effect on the quality of the finished products, above all if for reasons of productivity one does not want to wash the powder after coagulation of the dispersion. The emulsifier is among the ingredients which can have an effect on the quality of the finished products; the two most often used types of emulsifiers for the polymerization of tetrafluoroethylene are either the salts of perfluorated carboxylic acids or the salts of perfluorated sulfonic acids. The salts of carboxylic acids having a number of carbon atoms below 10 are products which are soluble in water, thus easy to employ, and they decompose thermally, below 300.degree. C., below the point of fusion (melting point) of the PTFE, without leaving traces. The salts of sulfonic acids having a number of carbon atoms from C.sub.6 to C.sub.16 are products which generally are very slightly soluble in water at a temperature below 80.degree.-100.degree. C., with the exception of the lithium salt.
The most often used emulsifiers are fluorinated or chlorofluorinated salts, in particular sodium or ammonium salts of a perfluoromonocarboxylic acid having 6 to 10 carbon atoms, such as ammonium perfluoroctanoate. They are introduced in aqueous solution, either totally at the start of polymerization, or progressively during the course of polymerization, as it is described in French Pat. No. 1,550,055. Although these emulsifiers give good results, they present the disadvantage of being costly and difficult to prepare. That is the reason why salts of sulfonic acids and, more particularly, salts of perfluoroalkanesulfonic acids are often substituted for them. These salts of sulfonic acids, on the other hand, are difficult to use because of their poor solubility in aqueous medium, with the exception of the lithium salt.
It has thus been considered to use, as the emulsifier, the lithium salt of perfluoroalkanesulfonic acids and, more particularly, salts of the formula LiCF.sub.3 (CF.sub.2).sub.n SO.sub.3, with "n" being a whole number between 5 and 16. It was, however, not possible to usefully employ these products since, being unstable at a temperature as low as 350.degree. C., they become degraded, leaving black traces in the PTFE.